<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			.a{
				width: 10px;
				height: 10px;
				background: red;
			}
			.b{
				width: 100px;
				height: 100px;
				background: blue;
			}
			.c{
				width: 1000px;
				height: 1000px;
				background: yellow;
			}
		</style>
	</head>
	<body>
		<select>
			<option value="a">方案1</option>
			<option value="b">方案2</option>
			<option value="c">方案3</option>
		</select>
		<button onclick="change()">改变</button>
		
		<div></div>
		
		<script>
			let op = document.querySelector('div');
			let set = document.querySelector('select');
			function change(){
				op.className = set.value;
			}
		</script>
	</body>
</html>